1. Field of the Invention
The present invention relates to a branch circuit configured to separate communication signals in a plurality of frequency bands and to a branch cable including the branch circuit.
2. Description of the Related Art
In recent years, mobile communication terminal apparatuses, such as cellular phone terminals, need to have a communication function of utilizing many frequency bands, and at the same time, a reduction in the size of the apparatuses is required. Hence, it is difficult to provide many antenna devices for respective frequency bands, and it becomes important to share an antenna and provide a branch circuit for branching antenna signals to a plurality of communication circuits.
Japanese Unexamined Patent Application Publication No. 2005-184773 discloses a branch circuit for switching among three transmission/reception systems: Cellular, GPS, and PCS. FIG. 25 is a circuit diagram of a branch circuit disclosed in Japanese Unexamined Patent Application Publication No. 2005-184773. This branch circuit supports three bands. The three communication systems are a Cellular communication system (first frequency band: 800 MHz), a GPS communication system (second frequency band: 1500 MHz), and a PCS communication system (third frequency band: 1900 MHz). This signal separation circuit includes an ANT terminal connected to an antenna, a first terminal 1 for input/output of transmission/reception signals in the first frequency band, a second terminal 2 for input of reception signals in the second frequency band, and a third terminal 3 for input/output of transmission/reception signals in the third frequency band. A low pass filter LPF 5 is connected between the ANT terminal and the first terminal 1. A phase adjustment circuit 6 and a SAW filter 7 are connected between the ANT terminal and the second terminal 2. A high pass filter (HPF) 8 and a phase adjustment inductor 9 are connected between the ANT terminal and the third terminal 3.
In FIG. 25, the LPF 5 allows signals in the frequency band of the Cellular communication system to pass therethrough and attenuates signals in the GPS communication system and the PCS communication system. The phase adjustment circuit 6 is provided to increase the respective impedances in the first frequency band and the third frequency band between the ANT terminal and the second terminal 2.
In a mobile terminal including a communication circuit for performing voice communication and data communication and a GPS reception circuit, there is a case in which an RF circuit is configured in such a manner as to supply a GPS signal to the GPS reception circuit and supply communication signals for voice communication and data communication to a single communication circuit. In such a configuration, it becomes necessary to separate the antenna signals into a GPS reception signal and communication signals. In this case, the communication signals include a high-band communication signal and a low-band communication signal and the GPS signal is in an intermediate frequency band located between the respective bands of the high-band communication signal and the low-band communication signal.
However, the existing signal separation circuit illustrated in FIG. 25 is a so-called triplexer, which only branches signals in three frequency bands to respective three input/output units and, hence, such a circuit configuration in which signals are separated into respective frequency bands cannot be used.
A configuration illustrated in FIG. 26 may be thought of as an example of a branch circuit that can separate a communication signal, which includes a high-band communication signal and a low-band communication signal, and a GPS signal in an intermediate frequency band located between the respective bands of the high-band communication signal and the low-band communication signal. Referring to FIG. 26, a band elimination filter (BEF) 11 eliminates a GPS signal of 1.5 GHz. A SAW filter 28 blocks a high-band communication signal and a low-band communication signal, and allows the frequency band of a GPS signal to pass therethrough. A communication circuit 10 is connected to the output stage of the band elimination filter (BEF) 11 and a GPS receiver circuit 20 is connected to the output stage of the SAW filter 28.
However, a branch circuit having the configuration illustrated in FIG. 26 has the following problems.
FIG. 27A illustrates the impedance locus of the SAW filter 28 seen from a connection port P1 of an antenna 30 and FIG. 27B illustrates the impedance locus of the band elimination filter 11 seen from the connection port P1 of the antenna 30. FIG. 28 illustrates the frequency characteristics of an insertion loss (S21 of S-parameters) between the connection port P1 of the antenna 30 and an output port P2 of the band elimination filter 11. All the results were obtained by sweeping over the frequency range of 0.7 GHz to 2.7 GHz.
As can be seen from FIG. 27A and FIG. 27B, there is a frequency having the same phase in the same frequency band in the SAW filter 28 and the band elimination filter 11, and the low band of the eliminated band of the SAW filter 28 is on the short-circuit impedance side, such that a signal input from the antenna flows to the ground through the SAW filter 28. Hence, as illustrated by broken-line ellipses (left-hand side) in FIG. 28, an attenuation band is generated in a pass band on the low-band side from 1.5 GHz, which is the frequency band of a GPS signal. As a result, a low-band communication signal input to the communication circuit 10 illustrated in FIG. 26 is attenuated. In the example of FIG. 27A, the eliminated band of the SAW filter 28 is in a short-circuit impedance state in the low band, but the high band may be in a short-circuit impedance state or both bands may be in a short-circuit impedance state, depending on the design of the SAW filter. In the case where the high band is in a short-circuit impedance state, as illustrated by broken-line ellipses (right-hand side) in FIG. 28, an attenuation band is generated in the pass band of the high band, such that signals in the high band input to the communication circuit 10 illustrated in FIG. 26 are attenuated.